This invention relates to an electrostatographic reproducing machine, particularly but not exclusively a xerographic copier, including a development apparatus at which an electrostatic latent image is developed on an imaging member, and a transfer device which causes a developed toner image to be transferred from the moving imaging member to a copy sheet.
Conventionally, in the automatic xerographic process, a latent electrostatic image of an original to be reproduced is recorded upon an image retaining member and the image then made visible, or developed, by means of a finely divided particulate toner material. In reusable xerography, the developed toner image is generally transferred from the image retaining member to a copy sheet, such as paper or the like, and the image affixed thereto to form a permanent record of the original input scene information. Although a preponderance of the toner material comprising the developed image is transferred to the copy sheet, a small amount of residual toner is nevertheless invariably left behind on the image retaining member surface after the transfer operation. In order to restore the image retaining member to conditions suitable for reuse, the residual toner must be cleaned or removed from the image retaining member surface, for example by means of an elastomeric blade cleaner, before a new imaging cycle is instituted. Generally, the residual toner is collected in a chamber where it may either be stored for later disposal or recirculated for further use.
Recently there has been a move in the xerographic art towards including the photoreceptor together with other process means such as a charge corotron, a developement device, a transfer corotron and a cleaning device in a process unit in the form of a cassette. An example of such a cassette is described in U.S. Pat. No. 3,985,436. The use of a cassette of this kind enables the easy replacement of those parts of the xerographic machine which are most likely to deteriorate with use, especially the photoreceptor, but also the development and cleaning systems as well as the corotron wires.
A xerographic cassette of this kind is preferably made of low cost materials, so that it can be disposed of at the end of its useful life. Such a cassette typically has a housing made of a molded plastics material, which is usually electrically insulating. This is generally convenient because it enables the various components in the cassette, including the electrical components, to be electrically isolated from one another without the need for additional insulating elements.
A problem may arise however, because of the tendency of debris particles to become electrically charged and to accumulate in the development apparatus, especially at or around the exit of the development apparatus. The debris typically includes toner particles and agglomerates from the developer housing, as well as paper fibers and small paper pieces. In the case of paper, paper fibers may be scrubbed or dislodged from the surface by the mechanism which feeds and advances copy paper sheets towards the transfer station, for example a friction retard system paper feeder frequently used in xerographic copiers. Despite the use of a fibre trap in addition to the main (blade) cleaner, some paper debris usually finds its way into the developer housing, either by migrating backwards (i.e. in the direction opposite the direction of movement of the photoreceptor) from the transfer station, or, in the case where cleaned-off toner is recirculated, by being cleaned off the photoreceptor with the toner and recirculated to the development apparatus. The insulating housing inhibits discharge of the charged debris, so the charge may be retained for long periods of time, especially in conditions of low ambient humidity. The build-up of charge in this way gives rise to the problem that unwanted charges may transfer to the photoreceptor, causing random marks or spots to be developed out in the final image.